We humans do not live a sterile life, no matter how much Purel we use. Every surface, from our kitchen counters to our reusable grocery bags to our shiny tablets displaying our recipes is covered in microorganisms. In a culture where cleaning products boast about the number of bacteria they kill, we're conditioned to believe that all microbes are bad, but the reality is that the vast majority of bacterial life on earth is not a threat (as an aside, don't forget that killing 99.9% of 10 billion bacteria leaves 10 million behind). In fact, our guts are filled with microbes that aide our digestion and protect us from pathogens. Over the coming months on this blog, I'll be talking a lot about our friendly passengers and about how understanding and manipulating our gut ecosystem may be as important as the coming genomics revolution.

Then again, this stereotype isn't completely without merit. Disclosure: you probably shouldn't read this before dinner.

Cholera is Shitty

Among the potential symptoms of infectious disease, perhaps none is unpleasant as diarrhea. For those of us in the western world, the unpleasantness rarely exceeds a couple of uncomfortable days in the bathroom, but for most of the developing world, infectious diarrheal illness is a significant public health threat. An estimated 2-4 billion episodes occur worldwide, and these are most prevalent in infants, whose stomachs are not as acidic and are therefore less able to kill of potential invaders on the way in. Excessive effluence (I will quickly run out of euphamisms for this) leads to rapid dehydration, sometimes so quickly that fluids cannot be replenished quickly enough without an IV. Adding insult to poopy injury, in many cases the source of water used to combat the loss of fluids is the same contaminated source that carried the infection in the first place, introducing fresh new invaders to replace the ones that are being expelled.

The types of pathogens that cause diarrhea are diverse, but the reason for it is always the same: causing diarrhea is an incredibly efficient method of transmission. In 2010, after a massive Earthquake devestated Haiti, UN peacekeepers from Nepal set up camp on the banks of a tributary that flows into one of the main sources of drinking water for Haiti. They were trying to help, but they brought a deadly passenger - Vibrio cholerae - and their sanitation system was poorly constructed, spilling human excrement into the water during a heavy rain. That single contamination has led to an outbreak that has sickened 600,000 people (over 5% of the population) and killed about 8,000 in the past 3 years.

Once it reaches your gut, the cholera bacterium sets up shop and replicates wildly, consuming the nutruients passing through. As the population expands, the bacteria begin to produce cholera toxin (CT), which hijacks the transport system of the cells lining your gut, makes its way inside the cells and ramps up a chemical reaction that tricks the cell into releasing all of its ions. Under normal circumstances, the gut is capable of retaining 8-9 liters (about 2 gallons) of fluids, but this release of ions causes massive flow of water from behind the barriers of the gut. The good news is, this torrential flow often carries the offending microbes along with it. The bad news is, it's tough to replace that water quickly enough to avoid death by dehydration.

The other bad news: the released bacteria flow downstream, ready to infect the next hapless soul that drinks a glass of water.

Other Baddies

Getting into our guts is a time honored tradition for viruses, bacteria (like cholera), and parasites. We're constantly putting foreign things into our mouths, and once they get past the acid-bath in the stomach, the intestines are a warm and inviting place for microbes. Most of these bugs don't induce diarrhea in quite the same way, but they all do it in order to make the leap to a new host. Generally speaking, killing you is not the goal of a pathogen, but diarrheal illness nevertheless kills millions of people every year, half of which are children. The perverse irony here is that preventing and treating all of these diseases should be simple: all you need is clean water.

Over the coming months on this blog, I'll be talking a lot about how microbes invade our guts and how our bodies try to fight them off. I'll also be talking about the good guys that make their homes in our bodies almost from the moment we're born, and how we can tell the good ones apart. I hope you'll stick around, and jump into the comments, on google+ or on twitter to join the conversation.

The views expressed are those of the author(s) and are not necessarily those of Scientific American.

ABOUT THE AUTHOR(S)

Kevin Bonham

Kevin Bonham is a Curriculum Fellow in the Microbiology and Immunobiology department at Harvard Medical school. He received his PhD from Harvard, where he studied how the cells of the immune system detect the presence of infectious microbes. Find him on Google+, Reddit.

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